1. Field of the Invention
The present invention relates to an operational amplifier and a driver circuit using the same.
2. Related Art
Conventionally, as liquid crystal panels (electro-optic devices) used for electronic apparatuses such a's mobile phones, simple matrix liquid crystal panels and active matrix liquid crystal panels using switching elements such as thin film transistors (hereinafter abbreviated as TFT) are known.
The simple matrix method has an advantage that low power consumption can more easily be achieved compared to the active matrix method on the one hand, but has a disadvantage that multi-colored images or movies are difficult to display on the other hand. In contrast, the active matrix method has an advantage that it is suitable for displaying multi-colored images or movies on the one hand, but has a disadvantage that low power consumption is difficult to achieve on the other hand.
Further, in recent years, for portable electronic apparatuses such as mobile phones, needs for displaying multi-colored images or movies increase in order to provide high quality images. Accordingly, the conventionally used simple matrix liquid crystal panels have gradually been replaced with active matrix liquid crystal panels.
Regarding the active matrix liquid crystal panel, it is preferable to provide an operational amplifier (Op-Amp) functioning as an output buffer in a data line driver circuit for driving a data line of the liquid crystal panel.
A structure of an operational amplifier known to the public is shown in FIG. 13.
In this operational amplifier, an n-type driver transistor M10 is controlled by a p-type differential input circuit including p-type transistors M7, M8, n-type transistors M5, M6, and a current source CSb. Further, a p-type driver transistor M9 is controlled by an n-type differential input circuit including p-type transistors M1, M2, n-type transistors M3, M4, and a current source CSa.
Focusing on the n-type differential input circuit, the case in which the voltage of an input signal Vin is higher than the voltage of the output signal Vout is considered. In this case, since the impedance of the n-type transistor M4 becomes larger than that of the n-type transistor M3, the gate voltages of the p-type transistors M2, M1 rise, and the impedance of the p-type transistor M1 increases. Accordingly, the gate voltage of the p-type driver transistor M9 drops, and the p-type driver transistor M9 proceeds to be switched-on.
Focusing on the p-type differential input circuit, when the voltage of the input signal Vin is higher than the voltage of the output signal Vout, the gate voltages of the n-type transistors M5, M6 rise and the impedance of the n-type transistor M5 decreases because the impedance of the p-type transistor M8 becomes smaller than the impedance of the p-type transistor M7. Accordingly, the gate voltage of the n-type driver transistor M10 drops, and the n-type driver transistor M10 proceeds to be switched-on.
As described above, when the voltage of the input signal Vin is higher than the voltage of the output signal Vout, the p-type driver transistor M9 and the n-type driver transistor M10 operate so as to increase the voltage of the output signal Vout. Note that when the voltage of the input signal Vin is lower than the voltage of the output signal Vout, they operate reversely to what is described above. As a result of the above operations, the operational amplifier proceeds to be in the balanced state in which the voltage of the input signal Vin and the voltage of the output signal Vout is approximately the same.
However, in the p-type differential input circuit the input signal Vin is supplied to the p-type transistor M7 as the gate voltage, and in the n-type differential circuit the input signal Vin is supplied to the transistor M3 as the gate voltage. Therefore, as shown in FIG. 14, input dead zones where the voltage of the input signal Vin and the voltage of the output signal Vout cannot be made equal appear in a range R1 of the input signal Vin between the high voltage side of the power supply voltages VDD and VDD−|Vthp| (Vthp denotes the threshold voltage of the p-type transistor M7) and a range R2 thereof between the low voltage side of the power supply voltages VSS and VSS+Vthn (Vthn denotes the threshold voltage of the n-type transistor M3). This is because the n-type differential input circuit does not operate because the n-type transistor M3 is kept in the off-state with the input signal in the range R2 between the low voltage side of the power supply voltages VSS and VSS+Vthn, and the p-type differential input circuit does not operate because the p-type transistor M7 is kept in the off-state with the input signal in the range R1 between the high voltage side of the power supply voltages VDD and VDD−|Vthp|.
For example, the case in which a liquid crystal panel is driven with a depth voltage having maximum amplitude (VinR) of 5 volts in 64 levels of depth is considered. In this case, if the amplitude of 5 volts is narrowed in order to generate the depth voltage corresponding to each of depth levels, there is caused a problem in the tone expression. Therefore, the depth voltage having the maximum amplitude (VDDR) of 6.9 volts is generated with about 1.9 volts offset taking tolerances in the threshold voltage Vthp of the p-type transistor and the threshold voltage Vthn of the n-type transistor into consideration. Accordingly, if the voltage of the power supply system for the data line driver circuit is 5 volts, a step-up circuit is necessary to be provided to generate the depth voltage having amplitude of about 6.9 volts. If a charge pump circuit is adopted as the step-up circuit, a step-up transistor and a step-up capacitor are required, and further, a component layout in which the high voltage is considered is required causing an enlarged chip size, an increased mounting cost, and increased power consumption. In particular, since the manufacturing process for the 5 volt system as a logic power supply is not enough for a high withstand voltage transistor which can withstand applied voltage of 7 volts or higher and is necessary to be used, an increased cost of the manufacturing process is also caused.
Moreover, in the operational amplifier having a structure shown in FIG. 13, when the input signal Vin in the input dead zone is input, the p-type driver transistor M9 and the n-type transistor M10 become uncontrollable, and therefore, the through current cannot be controlled to be reduced. Accordingly, it is problematical that the stability of the circuit is degraded and the power consumption is increased.
The present invention addresses the above technical problem and has an advantage of providing an operational amplifier and a driver circuit using the same, the operational amplifier being low cost, of low power consumption, and having high drive capacity.